Ball screw device

ABSTRACT

The invention relates to a ball screw device in which a screw shaft and a nut member are threadedly engaged with each other through a number of balls. In such a ball screw device, an inner surface of a cover plate contacting an end surface of a nut main body is provided with a projecting return piece adapted to be fit-engaged with a recess cut in an inner peripheral surface of the nut main body, the return piece being provided with a scoop portion for dislodging the balls from the ball rolling groove of the screw shaft and a direction switching passage for guiding the dislodged balls to the entrance of a ball return hole. The projecting return piece is divided into a first piece formed integral with the cover plate and a second piece fixed to the first piece by a curved plane including the center line of the direction switching passage.

TECHNICAL FIELD

[0001] The present invention relates to a ball screw device in which ascrew shaft and a nut member are threadedly engaged with each otherthrough the intermediation of a number of balls and, more particularlyto an improved ball screw device in which the nut member is equippedwith a so-called end cap type ball circulation structure.

BACKGROUND ART

[0002] Conventionally, a ball screw device in which a screw shaft and anut member are threadedly engaged with each other through theintermediation of a number of balls is well known. In such a ball screwdevice, a spiral ball rolling groove formed in an outer peripheralsurface of the screw shaft and a spiral load rolling groove formed in aninner peripheral surface of the nut member face each other to form aload passage, with balls effecting load application between the screwshaft and the nut member while rolling in this load passage. Further,the nut member is equipped with a non-load passage for circulating ballshaving rolled through the load passage back to the load passage. As thescrew shaft and the nut member make relative rotation, the ballsendlessly circulate from the load passage to the non-load passage, andfrom the non-load passage to the load passage.

[0003] Ball screw devices can be classified into several types accordingto the structure of the non-load passage, including a so-called end captype ball screw device. Examples of the end cap type ball screw deviceare disclosed in JP o6-201013 A and WO97/48922, the former of which isshown in FIG. 16. In this ball screw device, a nut member 100 iscomposed of a nut main body 102 in which a load rolling groove 101 asmentioned above is formed and a pair of end caps 103 attached to theaxial end surfaces of the nut main body 102. More specifically, a ballreturn hole 104 parallel to the axial direction is formed in the nutmain body 102, and the end caps 103 attached to the end surfaces of thenut main body 102 have scoop portions 108 for dislodging balls 107 froma ball rolling groove 106 of a screw shaft 105 and direction switchingpassages 109 for guiding the dislodged balls 107 to an entrance of theball return hole 104. By fixing the end caps 103 to the nut main body102, the ball return hole 104 and the direction switching passages 109are communicatingly connected together to complete a non-load passagefor the balls 107. In addition, the non-load passage of this end captype device circulates the balls from one to the other end of the nutmain body, so that the device has adaptability to an increase in thetotal length of the nut main body, which is advantageous when the leadof the ball rolling groove formed in the screw shaft is fast. Thus, thisend cap type ball screw device is most suitable for linearly guiding amovable body, such as a table, at high speed.

[0004] However, as described above, in the conventional end cap typeball screw device, the end caps have scoop portions for dislodging theballs from the ball rolling groove of the screw shaft and directionswitching passages for guiding the dislodged balls to the ball returnhole, so that the thickness of the end caps must be rather large. Thus,when the end caps are fixed to the end surfaces of the nut main body,the total length of the nut member is rather large for the magnitude ofthe load applied, thus making it impossible to achieve a reduction inthe size of the nut member.

[0005] To reduce the thickness of the end caps, it is expedient if thedirection switching passages 109 in the end cap 103 extend in adirection perpendicular to the axial direction of the screw shaft 105 asin the case of the conventional ball screw device shown in FIG. 16.However, the ball rolling groove 106 of the screw shaft 105 is formedspirally, and the balls 107 rolling in this ball rolling groove 106 alsohave a speed component in the axial direction of the screw shaft 105.Thus, when the direction switching passages 109 extend in theabove-mentioned direction, the advancing direction of the balls 107dislodged from the ball rolling groove 106 is changed forcibly, with theresult that the balls 107 repeatedly collide with the end caps 103.Thus, when the balls 107 are circulated at high speed, there is a fearof the end caps 103 being damaged. Further, a great resistance isoffered to the circulation of the balls 107, which leads to a limitationin the high speed feeding of the movable body. Further, the balls 107and the end caps 103 generate collision sound, which leads to a problemof noise during use.

DISCLOSURE OF THE INVENTION

[0006] The present invention has been made in view of the above problemsin the prior art. It is an object of the present invention to provide aball screw device which allows minimizing the total length of the nutmember and which can guide the balls dislodged from the ball rollinggroove smoothly to the ball return hole, thus generating minimum noiseeven when the balls are circulated at high speed and proving itself mostsuitable for the feeding of a movable body, such as a table.

[0007] To achieve the above object, in the ball screw device of thepresent invention, a recess is formed in the inner peripheral surface ofthe nut main body, and a return piece is fit-engaged with this recess,the return piece being accommodated in the recess without protrudingfrom the axial end surface of the nut main body. The recess is formed soas to be open in the axial end surface of the nut main body and at aposition corresponding to each of the end portions of the spiral loadrolling groove formed in the inner peripheral surface of the nut mainbody. Further, the return piece is equipped with a scoop portion fordislodging the balls from the ball rolling groove of the screw shaft anda direction switching passage for guiding the dislodged balls to theentrance of the ball return hole. By fit-engaging the return piece withthe recess, the balls having rolled through the load rolling groove ofthe nut main body roll into the direction switching passage by way ofthe scoop portion. Further, mounted to the nut main body is a pipemember forming a ball return hole substantially parallel to the screwshaft. When the return piece is fit-engaged with the recess of the nutmain body, the ball return hole of the pipe member and the directionswitching passage of the return piece are connected with each other,forming an endless circulation route for the balls. Further,substantially donut-shaped cover plates are fixed to the axial endsurfaces of the nut main body, with the cover plates covering the returnpiece fit-engaged with the recess of the nut main body.

[0008] Thus, in the ball screw device of the present invention, theendless circulation route for the balls is formed solely by fit-engagingthe return piece with the recess of the nut main body. Further, thereturn piece is accommodated in the recess of the nut main body and doesnot axially stick out from the nut main body, so that the total axiallength of the nut member threadedly engaged with the screw shaft throughthe balls is the same as that of the nut main body, thus making itpossible to minimize the total length of the nut member.

[0009] In particular, for the balls dislodged from the ball rollinggroove of the screw shaft to roll into the direction switching passage,it is important that the screw shaft side entrance region of thedirection switching passage be contiguous with the load rolling grooveof the nut main body. For that purpose, it is desirable for thedirection switching passage to be formed so as to be matched with thedirection of an extension of the ball rolling groove of the screw shaftin the tangential direction of the screw shaft. That is, the screw shaftside entrance region of the direction switching passage is formed so asto be inclined by the lead angle of the ball rolling groove with respectto a plane perpendicular to the axial direction of the ball shaft. Evenin the case in which the direction switching passage is thus formed soas to be inclined with respect to a plane perpendicular to the axialdirection of the screw shaft, the return piece in which the directionswitching passage is formed is accommodated in the nut main body, sothat the cover plate may simply be in the form of a plate, and there isno fear of an increase in the total length of the nut member completedby mounting the cover plate.

[0010] The return piece may be formed of metal or synthetic resin. Whilea metal return piece is superior from the viewpoint of strength withrespect to use over a period of time, a return piece of synthetic resinis superior from the viewpoint of reducing the noise generated when theballs roll.

[0011] Further, since the return piece is equipped with a scoop portionand a direction switching passage, the return piece has a complicatedconfiguration, and from the viewpoint of producing it easily, it isdesirable to adopt a construction in which the return piece is dividedinto a first piece and a second piece by a curved plane including acenter line of the direction switching passage, the return piececonsisting of a combination of these pieces.

[0012] Further, while the return piece may be singly fit-engaged withthe recess of the nut main body, it is also possible to form the returnpiece integrally with the cover plate, fit-engaging the return pieceprotruding from the cover plate with the recess of the nut main bodywhen the cover plate is mounted on the end surface of the nut main body.

[0013] On the other hand, in the screw shaft side entrance of thedirection switching passage, there is provided a scoop portion fordislodging balls from the ball rolling groove of the shaft screw. Fromthe viewpoint of guiding the balls smoothly to the direction switchingpassage without allowing them to collide with the scoop portion, it isdesirable for the scoop portion to protrude slightly on the shaft screwside with respect to the inner peripheral surface of the nut main bodyand for the direction switching passage to be formed in an arcuateconfiguration in conformity with the inner peripheral surface of thenut. By forming the scoop portion in such a configuration, the ballsrolling in the ball rolling groove of the screw shaft are held on eitherside as they are dislodged from the ball rolling groove, and roll asthey are into the direction switching passage, so that there is no fearof the balls colliding with the scoop portion, and even when the ballsare circulated through the endless circulation path at high speed, noexcessive force is applied to the return piece. Further, it is alsopossible to minimize noise generation.

[0014] In dividing the return piece into the first piece and the secondpiece by a curved plane including the center line of the directionswitching passage, the scoop portion may be divided between the firstpiece and the second piece or may be formed only in either the firstpiece or the second piece. In the former case, the direction switchingpassage is completely divided into the first piece and the second piece,including the scoop portion, so that the configuration of the firstpiece and the second piece is simplified, thus facilitating itsproduction. In contrast, in the latter case, in which the scoop portionis formed in either the first piece or the second piece, it isimpossible to completely divide the direction switching passage into thefirst piece and the second piece, and the configuration of the firstpiece and the second piece is somewhat complicated. On the other hand,it is advantageously possible to shape the scoop portion with highaccuracy.

[0015] Further, from the viewpoint of preventing the balls fromcolliding with each other in the endless circulation route to smoothenthe circulation of the balls and to prevent noise generation as much aspossible, it is desirable to provide spacers of synthetic resin betweenthe balls adjacent to each other. Considering that such spacers are tobe provided between the balls, it is not desirable for the rollingdirection of the balls in the endless circulation route to changeabruptly. When there is such a change, there is a fear of the spacersbeing dislodged from between the balls. Thus, from this viewpoint, it isdesirable for the screw shaft side entrance region of the directionswitching passage to be matched with the direction of an extension ofthe load rolling groove in the tangential direction of the innerperipheral surface of the nut main body. Further, it is desirable forthe scoop portion to be formed by cutting out the direction switchingpassage in an arcuate configuration in conformity with the innerperipheral surface of the nut.

[0016] Furthermore, according to the present invention, a syntheticresin pipe body is mounted to the nut main body to form a ball returnhole, and when the return piece in which the direction switching passageis formed of synthetic resin, the non-load passage for endlesscirculation of the balls is entirely covered with synthetic resin, sothat it is possible to minimize noise generation during the circulationof the balls as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a side sectional view of a ball screw device accordingto a first embodiment of the present invention;

[0018]FIG. 2 is a front view (partially cutaway sectional view) of theball screw of the first embodiment;

[0019]FIG. 3 is an exploded view of a nut member according to the firstembodiment;

[0020]FIG. 4 is a front view of a nut main body according to the firstembodiment;

[0021]FIG. 5 is a sectional view taken along the line V-V of FIG. 4;

[0022]FIG. 6 is a side view of a cover plate according to the firstembodiment;

[0023]FIG. 7 is a view seen from a direction of the arrow VII of FIG. 6;

[0024]FIG. 8 is a view seen from a direction of the arrow VIII of FIG.6;

[0025]FIG. 9 is a sectional view taken along the line IX-IX of FIG. 8;

[0026]FIG. 10 is a side view (partially cutaway sectional view) of areturn piece according to the first embodiment;

[0027]FIG. 11 is a bottom view of the return piece of the firstembodiment;

[0028]FIG. 12 is a view illustrating in sequence how a ball is dislodgedfrom a ball rolling groove of a screw shaft;

[0029]FIG. 13 is an exploded perspective view of a nut member of a ballscrew device according to a second embodiment of the present invention;

[0030]FIG. 14 is a perspective view of a return piece according to thesecond embodiment;

[0031]FIG. 15 is an exploded perspective view of the return piece of thesecond embodiment; and

[0032]FIG. 16 is a sectional view of a ball screw device as disclosed inJP 06-201013 A.

DESCRIPTION OF REFERENCE NUMERALS

[0033]1 . . . screw shaft, 2 . . . ball, 3 . . . nut member, 4 . . . nutmain body, 5 . . . cover plate, 6 . . . pipe member, 9 . . . secondpiece, 10 . . . ball rolling groove, 20 . . . spacer, 42 . . . loadrolling groove, 47 . . . recess, 53 . . . return piece, 54 . . . firstpiece, 55 . . . scoop portion, 56 . . . direction switching passage, 60. . . ball return hole

BEST MODE FOR CARRYING OUT THE INVENTION

[0034] A ball screw device of the present invention will now bedescribed in detail with reference to the accompanying drawings.

[0035]FIGS. 1 and 2 show a ball screw device according to a firstembodiment of the present invention. This ball screw device is composedof a screw shaft 1 whose outer peripheral surface has a spiral ballrolling groove 10 formed at a predetermined lead, and a nut member 3threadedly engaged with the screw shaft 1 through the intermediation ofa number of balls 2 and equipped with an endless circulation route forthe balls 2, wherein the nut member 3 makes a movement in the axialdirection of the screw shaft 1 through relative rotation of the screwshaft 1 and the nut member 3. Further, the screw shaft 1 has two streaksof ball rolling grooves 10, and the nut member 3 also has two endlesscirculation routes of the balls 2.

[0036] In this ball screw device, spacers 20 are provided between theballs 2 adjacent to each other in the endless circulation routes, thuspreventing the balls 2 from coming into direct contact with each other.The spacers 20 are disc-like members formed of synthetic resin, eachhaving on either side of the disk-like member a spherical seat on whichthe spherical surface of each ball 2 is to be seated.

[0037]FIG. 3 is an exploded view of the nut member 3. As shown in thedrawing, the nut member 3 is composed of a nut main body 4 formed ofsteel, return pieces 53 fit-engaged with the nut main body, a pair ofcover plates 5 of synthetic resin fixed to the front and rear endsurfaces of the nut main body 4, and a pair of pipe members 6 sandwichedbetween the cover plates 5 and fixed to the nut main body 4, whereinendless circulation routes for the balls 2 are formed by fixing thereturn pieces 53, the cover plates 5, and the pipe members 6 to the nutmain body 4. Further, attached to the cover plates 5 are labyrinth seals7 for preventing dust from entering the interior of the nut member 3,with the labyrinth seals being fixed to the cover plates 5 by a sealpresser plates 8 fixed to the cover plates 5 by means of screws. FIG. 3only shows the cover plate 5 mounted to one side of the nut main body 4.

[0038]FIGS. 4 and 5 show the nut main body 4. This nut main body 4 isformed as a cylinder having at its center a through-hole 40 for thescrew shaft 1, and a flange portion 41 for fixing the nut main body 4 toa movable body such as a table protrudes from the outer peripheralsurface thereof. Further, in the inner peripheral surface of thethrough-hole 40, there are formed spiral load rolling grooves 42 facingthe ball rolling grooves 10 of the screw shaft 1, with the balls 2rolling between the ball rolling grooves 10 and the load rolling grooves42 while applying load. The flange portion 41 has through-holes 43serving as bolt holes for fixing the nut main body 4 to the movablebody.

[0039] The axial end surfaces of the nut main body 4 constitute mountingsurfaces 44 of the cover plates 5, and tap holes 45 are formed in themounting surfaces 44 for threaded engagement of mounting screws for thecover plates 5. Between these mounting surfaces 44, there are formedpipe mounting holes 46 so as to axially extend through the nut main body4, and the above-mentioned pipe members 6 are fit-engaged with the pipemounting holes 46. The pipe members 6 are formed as cylinders with ballreturn holes 60, and the balls 2 under no load released from the loadroll into through the ball return holes 60 in the axial direction of thenut main body 4. Further, in order to prevent, as much as possible,noise generation due to the contact between the balls 2 rolling at highspeed through the ball return holes 60 and the pipe members 6, the pipemembers 6 are formed of synthetic resin.

[0040] In conformity with the two streaks of ball rolling grooves 10formed in the screw shaft 1, two pipe mounting holes 46 are provided soas to be spaced apart from by 180 degrees and opposed to each other withthe through-hole 40 therebetween. Recesses 47 for accommodating thereturn pieces 53 are formed at the ends of the pipe mounting holes 46open in the mounting surfaces 44 for the cover plates 5. As shown inFIG. 4, these recesses 47 are cut out so as to extend tangentially fromthe through-hole 40 of the nut main body 4, connecting the through-hole40 to the pipe mounting holes 46. The ends of the two streaks of loadrolling grooves 42 formed in the inner peripheral surface of the nutmain body 4 are divided by the recesses 47 immediately before they reachthe cover plate mounting surfaces 44.

[0041]FIGS. 6 through 8 show the cover plate 5. The cover plate 5 isformed as a substantially donut-shaped member covering the mountingsurface 44 on the nut main body side; it is prepared by injectionmolding of synthetic resin. On the outer side of the cover plate 5,there is formed an annular groove 50 for mounting the labyrinth seal 7around the through-hole 51, and there are formed screw holes 52 forfixing the seal presser plate 8 on the outer side of the annular groove50. Further, mounting holes 59, through which the mounting screws formounting the cover plate 5 to the nut main body 4 are to be passed,extend through the cover plate 5.

[0042] Further, on the inner side of the cover plate 5, there protrudethe return pieces 53 to be fit-engaged with the recess of the nut mainbody. The return pieces are composed of first pieces 54 formed so as toprotrude from the inner side of the cover plate 5 and second pieces 9(See FIGS. 10 and 11) fixed to the first pieces 54 by screws. As shownin FIG. 3, by combining the first pieces 54 with the second pieces 9,the return pieces 53 are equipped with scoop portions 55 for dislodgingthe balls 2 from the ball rolling grooves 10 of the screw shaft 1 anddirection switching passages 56 for feeding the dislodged balls 2 intothe ball return holes 60 of the pipe members 6. The second pieces 9 areequipped with flanges 92 for accommodating hexagonal nuts (not shown).Bolts inserted from the cover plate 5 side are threadedly engaged withthe hexagonal nuts, whereby the second pieces 9 are fixed to the coverplate 5. Through-holes 58 through which the bolts are to be passed areformed adjacent to the first pieces 54, and through-holes 91 throughwhich the bolts are to be passed are formed adjacent to the secondpieces 9.

[0043] Inside these return pieces 53, the direction switching passages56 are curved so as to vary the rolling direction of the balls 2 by 90degrees or more, and one end of each of the direction switching passages56 is open on a side surface of the return piece 53 raised from thecover plate 5, the other end thereof being open at the forward endsurface of the return piece 53 protruding from the cover plate 5. Theformer opening is the screw shaft 1 side entrance, and the latteropening is the ball return hole 60 side entrance formed in the pipemember 6. An annular groove 61 is formed at the ball return hole 60 sideedge portion of each direction switching passage 56. When the coverplate 5 is attached to the nut main body 4, the forward end portions ofthe pipe members 6 are fit-engaged with the annular grooves 61. Due tothis arrangement, the direction switching passages 56 provided in thecover plate 5 and the ball return holes 60 formed in the pipe members 6are connected together with high accuracy, making it possible tosmoothen the rolling of the balls 2 between these passages.

[0044] Each of the return pieces 53 is divided by a curved planeincluding the center line of the direction switching passage 56 into thefirst piece 54 and the second piece 9. Thus, the first piece 54 and thesecond piece 9 respectively have grooves 57 and 90 with a semi-circularsection and opposed to each other, and these grooves 57 and 90 arecombined with each other to form the direction switching passage 56.Further, formed at the screw shaft 1 side entrance of each directionswitching passage 56 is a scoop portion 55 for dislodging the balls 2from the ball rolling groove 10 of the screw shaft 1, and this scoopportion 55 is also divided into the first piece 54 and the second piece9. This scoop portion 55 protrudes toward the screw shaft 1 sideslightly beyond the inner peripheral surface of the nut main body 4, andis arranged in correspondence with the gap between the inner peripheralsurface of the nut main body 4 and the outer peripheral surface of thescrew shaft 1. Further, the scoop portion 55 is formed by cutting thedirection switching passage 56 in an arcuate configuration in conformitywith the inner peripheral surface of the nut main body 4. Thus, thisscoop portion 55 is substantially the same configuration as obtained byobliquely cutting one end portion of a pipe. As the balls 2 advancesinwards from the inlet of the direction switching passage 56, the scoopportion 55 covers the balls 2 gradually from either side with respect tothe advancing direction, eventually accommodating the balls in thedirection switching passage.

[0045] The nut member 3 formed by the above components is assembled bythe following procedures. First, the pipe members 6 are inserted intothe pipe mounting holes 46 of the nut main body 4, and then the pair ofcover plate 5 with the second pieces mounted thereto are fixed to themounting surfaces 44 of the nut main body 4 by screws. When the secondpieces 9 are secured to the first pieces 54 formed integrally with thecover plates 5, there are formed the return pieces 53 equipped with thedirection switching passages 56 and the scoop portions 55 in the coverplates 5, and by fit-engaging these return pieces 53 with the recesses47 formed in the nut main body 4, it is possible to accurately performpositioning on the cover plates 5 with respect to the nut main body 4.Further, when the cover plates 5 are attached to the nut main body 4,the pipe members 6 previously inserted into the nut main body 4 are heldfrom both ends by the protrusions 53 of the cover plates 5, and the endportions of the pipe members 6 are fit-engaged with the annular grooves61 formed at the forward ends of the protrusions 53, whereby an endlesscirculation route for the balls 2 is completed in the nut member 3.

[0046] When the screw shaft 1 rotates, the balls 2 roll between the ballrolling grooves 10 of the screw shaft 1 and the load rolling grooves 42of the nut main body 4 while applying load, and are then brought into ano-load state before entering the direction switching passages 56provided in one return pieces 53. Then, the balls 2 are guided to thedirection switching passages 56 of the other return pieces 53 by way ofthe ball return holes 60 of the pipe members 6, and are returned fromthe direction switching passages 56 to the ball rolling grooves 10 ofthe screw shaft 1 again. Since the cover plates 5, the second pieces 9,and the pipe members 6 are all formed of synthetic resin, of the endlesscirculation route for the balls 2, the direction switching passages 56and the ball return holes 60, where the balls 2 roll under no load, areall covered with synthetic resin. Further, as described above, thespacers 20 of synthetic resin are provided between the balls 2 rollingone after another in the endless circulation route, so that there is nofear of the balls 2, which consist of steel balls, coming into contactwith each other. Thus, the only metal members that come into contactwith the balls 2 are the ball rolling grooves 10 of the screw shaft 1and the load rolling grooves 42 of the nut main body 4, so that, in thisball screw device, it is possible to minimize the noise generationduring the circulation of the balls 2 as far as possible.

[0047] Further, as shown in FIG. 1, in this ball screw device, when thescoop portion 55 dislodge the balls 2 from the screw shaft 1 at theentrance of the direction switching passage 56, the balls 2 are guidedin a direction matched with the lead angle α of the ball rolling groove10. Further, as shown in FIG. 2, the ball 2 are dislodged from the ballrolling groove 10 of the screw shaft 1 so as to move in the tangentialdirection of the screw shaft 1, and enter the direction switchingpassage 56. That is, the screw shaft 1 side entrance regions of thedirection switching passages 56 are formed so as to be matched with thedirections of tangential extensions of the ball rolling grooves 10 ofthe screw shaft 1, in other words, in conformity with the direction inwhich a cord spirally wound around the screw shaft 1 is unwound.

[0048] FIGS. 12(a) through 12(f) illustrate how the scoop portion 55formed in each return piece 53 detaches a ball 2 from the ball rollinggroove 10 of the screw shaft 1. As shown in these drawings, the screwshaft 1 side entrance region of the direction switching passage 56 isformed so as to be opposed to the ball rolling groove 10, and the scoopportion 55 detaches the rolling ball 2 from the ball rolling groove 10as the scoop portion 55 holds the ball gradually from either side,eventually accommodating the ball in the direction switching passage 56.Thus, the ball 2 being dislodged from the screw shaft 1 receives noexternal force that would forcibly change its rolling direction, and itis possible to smoothly dislodge the ball 2 from the ball rolling groove10 and guide the balls 2 to the direction switching passage 56. Further,the direction switching passages 56 are gently curved and communicatewith the ball returning holes 60, so that it is possible to dislodge theballs 2 smoothly from the ball rolling grooves 10 and guide them to thedirection switching grooves 56. Further, the direction switchingpassages 56 are gently curved and communicate with the ball return holes60, there is no fear of the balls 2 violently colliding with the innerwalls of the direction switching passages 56, making it possible tosmoothen the circulation of the balls 2 and to prevent noise generationas far as possible.

[0049] Then, in the ball screw device of this embodiment, theabove-described direction switching passages 56 and the scoop portions55 are provided in the return pieces 53 protruding from the cover plates5, and the return pieces 53 are fit-engaged with the recesses 47 formedin the nut main body 4, so that the donut-shaped cover plates 5themselves can be formed very thin, and it is possible to minimize thetotal axial length of the nut member 3. Further, even in the case inwhich the cover plates 5 are thus formed thin, it is possible to set theprotruding amount of the return pieces 53, fit-engaged with the nut mainbody 4, with respect to the cover plates 5 large to some degree, so thatit is possible to form direction switching passages 56 gently curvedwith respect to such return pieces 53, making it possible to smoothenthe circulation of the balls 2 and to prevent as far as possible thegeneration of noise due to the circulation.

[0050]FIG. 13 shows a ball screw device according to the secondembodiment of the present invention.

[0051] While in the first embodiment described above the first pieces 54forming the return pieces 53 are formed integrally with the cover plates5, in the second embodiment, the first pieces 54 and the cover plates 5are separated from each other, and the return pieces 53 are fit-engagedwith the recesses 47 of the nut main body 4 without fixing the coverplates 5 to the nut main body 4. Otherwise, this embodiment is of thesame construction as the first embodiment, so that the components thatare the same as those of the first embodiment are indicated by the samereference numerals in FIG. 13, and a detailed description of suchcomponents will be omitted.

[0052]FIG. 14 is a perspective view showing the return piece 53 adoptedin the second embodiment. FIG. 15 is a perspective view showing thereturn piece 53 as divided into the first piece 54 and the second piece9. While the first piece 54 and the second piece 9 may be formed bymachining metal pieces, due to their complicated configuration, it isdesirable to produce them by metal injection molding (MIM). Of course,as in the first embodiment, it is also possible to produce them byinjection molding of synthetic resin.

[0053] The return piece of the second embodiment is also divided intothe first piece 54 and the second piece 9 by a curved plane includingthe center line of the direction switching passage 56. Unlike the firstembodiment, this embodiment adopts a construction in which the screwshaft 1 side entrance of the direction switching passage 56 is formed inthe second piece 9 without being divided. That is, the scoop portion 55provided at the screw shaft 1 side entrance of the direction switchingpassage 56 is formed exclusively in the second piece 9. Thus, ascompared with the return piece of the first embodiment, the return pieceof this embodiment allows the scoop portion 55 to be formed with higheraccuracy, which makes it possible to perform the operation of dislodgingthe balls 2 from the ball rolling groove 10 of the screw shaft 1 in amanner so much the more stable.

[0054] Further, the screw shaft 1 side entrance of the groove 90 formedin the second piece 9 is covered with a guide member 93 for guiding thespacers 20, provided between the balls 2, to the direction switchingpassage 56 together with the balls 2. When the first piece 54 and thesecond piece 9 are coupled with each other, the guide member is situatedat the screw shaft 1 side entrance of the direction switching passage56.

[0055] As described above, in the ball screw device of the presentinvention, the endless circulation route for the balls is formed solelyby fit-engaging the return pieces with the recesses in the nut mainbody. Further, the return pieces are accommodated in the recesses of thenut main body and do not axially stick out of the nut main body, so thatthe cover plates themselves may simply consist of plate-like members,and even when the cover plates are mounted to the end surfaces of thenut main body to form the nut member, it is possible to minimize thetotal length of the nut member, thus making it possible to achieve areduction in the size of the nut member. Further, regarding the returnpieces fit-engaged with the nut main body, there are no limitations dueto the reduction in size, and it is possible to freely form thedirection switching passages, so that the balls dislodged from the ballrolling grooves of the screw shaft can be smoothly guided to the ballreturn holes, making it possible to minimize noise generation even whenthe balls are circulated at high speed. Thus, the ball screw of thepresent invention is optimum for the high-speed feeding of a movablebody, such as a table.

[0056] In the first piece 54 and the second piece 9, there are formedthrough-holes 92 for fixing the return piece 53, obtained by couplingthem together, to the nut main body 4. As shown in FIG. 13, by usingscrews 95 inserted into the through-holes 92, it is possible to securethe return pieces 53 in the recesses 47 of the nut main body 4. Thus, byfit-engaging the return pieces 53 with the recesses 47 of the nut mainbody 4, and by fixing them by means of the screws 95, it is possible tofix the return pieces 53 to the nut main body 4 accurately and firmly.

[0057] After the completion of the fixation of the return pieces 53 tothe nut main body 4, the cover plates 5 are fixed to the plate mountingsurfaces 44 of the nut main body 4 so as to cover the return pieces 53.These cover plates 5 are used for the purpose of fixing the labyrinthseals 7 to the nut main body 4 and sealing the gap between the nut mainbody 4 and the screw shaft 1. The cover plates 5 need not to be sostrong as the return pieces 53, so that the cover plates 5 may as wellbe molded from synthetic resin even if the return pieces 53 are formedof metal.

[0058] Further, also in the ball screw device of the second embodiment,constructed as described above, the return pieces 53 are fit-engagedwith the recesses 47 formed in the nut main body 4, so that there is noneed to form the direction switching passages 56 in the cover plates 5,thus making it possible for the cover plates 5 themselves to be formedvery thin. Further, in this ball screw device, two streaks of endlessball circulation routes are formed, and it is necessary to fit-engagetwo return pieces 53 to one end surface of the nut main body 4 and tworeturn pieces 53 to the other end surface thereof, that is, four returnpieces 53 in total. In the above-described first embodiment, two returnpieces 53 are integrally formed with one cover plate 5. Thus, if theaccuracy in the formation of the recesses 47 with respect to the nutmain body 4 is rather low, the low positioning accuracy for the returnpieces 53 with respect to one side of the nut main body 4 may result ina deterioration in the positioning accuracy for the return pieces 53with respect to the other side. However, in the ball screw device of thesecond embodiment, the return pieces 53 are individually fit-engagedwith the recesses 47 of the nut main body 4, so that the respectivepositioning accuracies for the return pieces 53 with respect to the nutmain body 4 are independent of each other, and it is possible tofit-engage the return pieces 53 with the nut main body 4, dependingsolely on the accuracy in the formation of the recesses 47 in the nutmain body 4. This helps to realize a still more smooth endlesscirculation of the balls.

INDUSTRIAL APPLICABILITY

[0059] As described above, in the ball screw device of the presentinvention, the endless circulation route for the balls is formed solelyby fit-engaging the return pieces with the recesses in the nut mainbody. Further, the return pieces are accommodated in the recesses of thenut main body and do not axially stick out of the nut main body, so thatthe cover plates themselves may simply consist of plate-like members,and even when the cover plates are mounted to the end surfaces of thenut main body to form the nut member, it is possible to minimize thetotal length of the nut member, thus making it possible to achieve areduction in the size of the nut member. Further, regarding the returnpieces fit-engaged with the nut main body, there are no limitations dueto the reduction in size, and it is possible to freely form thedirection switching passages, so that the balls dislodged from the ballrolling grooves of the screw shaft can be smoothly guided to the ballreturn holes, making it possible to minimize noise generation even whenthe balls are circulated at high speed. Thus, the ball screw of thepresent invention is optimum for the high-speed feeding of a movablebody, such as a table.

1. A ball screw device, characterized by comprising: a screw shafthaving in its outer peripheral surface a spiral ball rolling groove; ametal nut main body formed substantially as a cylinder with athrough-hole through which the screw shaft is to be passed, the metalnut main body being threadedly engaged with the screw shaft through anumber of balls, having in its inner peripheral surface a spiral loadrolling groove facing the ball rolling groove of the screw shaft to forma load ball passage, and having in its inner peripheral surface a recesscut out in correspondence with both ends of the load rolling groove; asynthetic resin pipe member mounted in the nut main body and forming aball return hole substantially parallel to the screw shaft; a returnpiece fit-engaged with the recess of the nut main body, the return piecebeing equipped with a scoop portion for dislodging the balls from theball rolling groove of the screw shaft and a direction switching passagefor guiding the dislodged balls to an entrance of the ball return hole,and cooperating with the pipe member to form an endless circulationroute for the balls; and a cover plate formed substantially in adonut-like shape and fixed to each of both axial end surfaces of the nutmain body to cover the return piece fit-engaged with the recess of thenut main body.
 2. A ball screw device according to claim 1,characterized in that the screw shaft side entrance region of thedirection switching passage is bored in conformity with a direction ofan extension of the ball rolling groove of the screw shaft in atangential direction of the screw shaft.
 3. A ball screw deviceaccording to claim 2, characterized in that the scoop portion protrudestoward the screw shaft slightly beyond the inner peripheral surface ofthe nut main body, and that the direction switching passage is formed bycutting out the direction switching passage in an arcuate configurationin conformity with the inner peripheral surface of the nut main body. 4.A ball screw device according to claim 1, characterized in that thereturn piece is divided into a first piece and a second piece by acurved plane including a center line of the direction switching passage.5. A ball screw device according to claim 4, characterized in that thescoop portion is divided into a first piece and a second piece, and thatthe scoop portion is completed at a screw shaft side entrance of thedirection switching passage by coupling the first piece and the secondpiece with each other.
 6. A ball screw device according to claim 4,characterized in that the scoop portion is formed in one of the firstpiece and the second piece.
 7. A ball screw device according to one ofclaims 4 and 6, characterized in that the first piece is formedintegrally with the cover plate, whereas the second piece is fixed tothe first piece.
 8. A ball screw device according to claim 1,characterized in that an annular groove to be fit-engaged with an endportion of the pipe member is formed at a ball return hole side edgeportion of the direction switching passage, and that, when the returnpiece is fit-engaged with the recess of the nut main body, the pipemember is secured in position to the nut main body.